Fluid pressure control system for variable pitch propellers



Jun I5, 1954 R. c. TRESEDER 2,681,116

' FLUID PRESSURE CONTROL SYSTEM I FOR VARIABLE PITCH PROPELLERS Filed May 7, 1949 2 Sheets-Sheet l ILL/ [8/ 3X 54 50\ I A Y 4 //v VEN TOR I v 44w 6'15 ATTORNE YS' June 15, 1954 R. c. TRESEDER 2,681,116

FLUID PRESSURE CONTROL SYSTEM 1 FOR VARIABLE PITCH PROPELLERS Filed May 7, 1949 2 Sheets-Sheet 2 Q. E l Q Q wsfik @QE T. Sn NNR w v o Q. g 1% w\ \NO W NS Q E P E .0 QM w\ m. o Q R y g Q s g g (R @Q 3 Q WW g 0 3 a Q Q w 3 H n 1 E E Q .1 W t E Q 8% s I 1 .@M\ %Q\ RN DRN.

' INVENTOR. EKMGM BY M MQZ-r m3 ATTORNEYS Patented June 15, 1954 UNITED STATES AT ENT OFFICE FLUID PRESSURE CONTROL SYSTEM'FOR VARIABLE PITCH PROPELLERS Robert C. Treseder, Dayton, Ohio, assignor to General .Motors Corporation, Detroit, .Mich., a

corporation of Delaware Application May 7, 1949, Serial No.'92,043

11 Claims. 1

-iThisinvention accomplishes continuous con- .trol of adjustable mechanism by equal applications of. .actuatingforces alternately to opposite 1 sides of a movable member during status maintenance, andunequal applications of actuating .forces alternately to opposite sides of the movable. member duringstatus changes. Fluid under pressure motivates the movable member when applied by a double acting valve connecting fluid pressure to one side of a servo mechanism and .drain to the other side. Opposed solenoids when alternately energized control movement of the valve, and a rotating drum or other device with interrupting contacts effects alternate energization of the solenoid at some predetermined frequency or periodicity.

A pick-off contact whose shiftable position is related to the governed speed of rotation of the drum alternately connects the solenoids with a source of electric current for equal periods of time during status maintenance, and differentially varies the length of solenoid energization during status changes. The surface of the rotating drum provides two contacts each of which have tapering portions that-extend between like portions of the'other but insulated therefrom, so that sequential engagement with a movable contact arm occurs on rotation of the drum.

The pick-off contact rubbing on the drum at the mid-point engages equal portions of the two contacts to effect equal energization of the solenoids, and when shifted to one side of the middle of the drum successively" engage contacts for unequal periods of time. The dwell on one contact will be shortened as the dwell on the other is lengthened. The change in solenoid energization is therefore differentiaL' Manual means for shifting the point of pick ofi 'andgovernor operated means for restoring the status maintenance condition, should there occur disturbing forces tending to change a predetermined con-- quency or periodicity'of valve reciprocationthere Willbe no objectionable vibrationof the-adjustable mechanism. :For the majority of applications of the system of control,- reciprocation of the 'valve' at about-twenty cycles-per second (C. P. S.) has been founclto'be satisfactory; but

- the rate may bealtered to suit specific-conditions. During status change conditions, that is, when the adjustable mechanismisbeing changed to a different status, the drum 130131365 1301 deliver unequal electrical ,pulses.- to the opposed solenoids, that is, a longenpulse-to-one and a shorter pulse to theother; resulting in a-recipro- :tion' as. the status maintained-condition; and

movable to a different .position or at a specific rate of speed as the status change condition. l On the other ;hand;and as :generally disclosed-herein,

the..adjustable mechanism may be a: rotatable ."device whoseselected speed of rotation isthe status maintained condition, and whose.-.modification to a different selected speed or condition is the status .changecondition.

Further objects and advantages'of the-present invention will be apparent from the following dition. make it possible to closely control the 1 adjustable mechanism at all-times.

During status maintenance condition the drum rotates to deliver substantially equal electrical pulses to the opposed solenoids, first one and then the other, resulting in a reciprocating movement of the double acting valve which alternately connects the fluid pressure source for equal periods of time with opposite sides ofthe servo-mechanisin' The" opposite sides of the servo-mechanism receiving equal force applications, there is no resultant movement of the adjustable mechanism, and thus,-' the 1. status "maintained condition." With asatisfactory-fre- "description; reference being "had to the accom- .panyingdrawings wherein a preferred embodiment of the present invention is clearly shown.

In the drawings:

Fig. 1 is a schematic view illustrating means,

.mechanism "and a method'of control'by the invention disclosed herein.

' Fig. 2 is a longitudinal sectional View of a rotary device, such as a variable pitchpropeller, embodying control means and -mechanism according to the instant invention.

. Referring first t'o Fig. 1, I0 indicatesaservomotor comprising a cylinder I 21 enclosing adouble acting piston- I4 that .divides the cylinder into a pair of expansion chambers l6 and I8, a' piston M- being mounted on a'rod '20 adaptedto-move a load device or adjust some particular mechanism here shown as a propeller blade, 22 mounted on a hub socket 24 to be driven by a shaft 26 in the direction of arrow A by means of a prime mover such as an engine or turbine (not shown). The expansion chambers I6 and I8 are hydraulically connected by passages 28 and 30 respectively with control ports 32 and 34 respectively in a valve sleeve 30. A pressuresupply port 38 opens by passage 40 from a fluid pressure pump 42 whose intake is at, It is designed that the pump 42 operate continuously so as to supply fluid under pressure thru the port 38 into the bore 46 of the sleeve 36, a relief valveAl being provided for safety purposes. Cooperable with the valve sleeve 36 is a plunger or rod 48 having lands 50 and 52 properly spacedl to coincidentally cover the ports 32 and 34 when the valve rod 43 is in the mid-position, or the port closi g position. At each endof the rod 48 there are armatures 54 and 55 immersed in the magnetic field of solenoid windings 58 and 60 respectively, each of which when energized is capable thru its action upon the associated armature in shifting the rod 48 axially of the sleeve 36 so as to open either of the ports 32 and 34 to pressure Y from thepassage 40 and the other port to drain.

Solenoid windings 58 and 60 are joined by a wire 62 and communicate by a common connection 63 with one pole of a current source, such as a battery indicated at 64. The opposite ends of 7 face of the drum but electrically insulated by a non-conducting boundary of insulation 84. The

surface of the drum is therefore covered by a succession of alternating contact portions 80 and I 82 of such extent as to be arcuately equally dimensioned along the middle of the drum such as indicated by the dash line "85. d

A constant speed source, such as a constant speed motor 88 energized by the source leads 90, Y and adjustable for speed by the control 92, drives 7 the drum contact 14 thru a shaft 94 in the direction indicated by the arrow B. A manual control 95 pivoted at 98 engages a collar 30 and is capable upon oscillation over a scale I02, of moving the drum I4 axially upon the shaft 94 of the constant speed device 88 such that a movable contact I04 remaining in aselected position on a slide rod I05 will engage at a shifted position upon'the surface of the drum I4. The movable contact I04 is electrically connected to the other pole of the battery 64 by means of a lead I08 such that upon closure of a control switch I09 circuits may be completed between the current source 64,

one of the contacts 80-82 and one of the solenoid windings 58-50, and back to the current f source 64. r

i The movable contact I04 is automatically shiftable on the guide rod I06, and hence along the will either expand or contract, which oscillates the lever I I4 and shifts the contact I04 along the guide I66. As an example, and assuming an 7 increase of speed,-the shaft 26 will rotate faster in the direction of arrow A, and the shaft I24 will rotate faster in the direction of arrow C, whichwill cause the speed responsive device I22 to expand and oscillate the lever H4 in a clockwise direction to shift the movable contact I04 toward the left in Fig. 1. Assuming that the movable contact I04 has heretofore been engaging the drum I4 at the midpoint along the line 86 at which the dwells upon the contacts 8082 are substantially equal, the left hand movement of th movable contact I04 will then differentially alter the successive engagement upon the contacts 80, 82 in such direction as to lengthen the dwell upon contact 80 and to shorten the dwell upon contact 82. Since every instance of contact Hi4 wiping over the rotarycontacts 80, 82 energizes the associated solenoids 58, 50 in proportion to the length of dwell on the contact elements, and since every engagement of contacts 89, 52 with I94 effects energization of one of the solenoids with a corresponding port opening, then the alternate port openings at 32, 34 will be in proportion to the length .of time of the corresponding contact engagement. Thereby, a succession of fluid pressure pulses is delivered to opposite sides of the piston in the servo motor. In consequence of this, solenoid 58 will be energized for a longer period of time than is the solenoid t0 energized. When the solenoid Kit is nergized the valve plunger 48 is moved to the right connecting pressure port 38 with control port 34, and opening control port 32 to drain. Fluid pressure will then be applied for a longer period of time in expansion chamber I8 than is applied to the expansion chamber IS, which gives rise to a resultant movement of the piston I4 toward the left which operates to rotate the blade 22 in such direction as to increase its pitch, and thereby thru its increase of load reduces the speed of the engine-propeller combination. Conversely the reduction of speed of the shaft 20 reflects thru' 7 I04 will effect a 50:50 pulse ratio can be varied by movement of lever 96 about pivot 98. For example, to increase the speed setting, the lever 96 is moved counterclockwise about pivot 98, as

length of the drum l4, since it is mounted upon i 'a carriage IIO actuated b'y a link II2 from a lever II4 pivoted at I I0 and engaging a collar H8 on a movable shaft element I20 of a speed responsive device I22 driven by a shaft I 24in the direction of arrow C by means of a pinion I 26 that isactuated by a gear element I28 from the propeller shaft element 26. In consequence, the speed, responsiv device I22is a functionof the viewed in Fig. 1, and by reason of its engagement with collar I00 of shaft S4, drum 14 will be moved to the left. Under these conditions in order for contact I04 to engage the medial portion of the drum I4 which represents an onspeed condition, carriage H0 must likewise move to the left along slide rod I06. Movement of carriage IIO to the left can only be effected by the speed responsive device I22 and its associated linkage I20, H8, H4 and H2 when .the device I22 responds to. an increase in the rotative speed of propeller shaft 26 as selected by the lever 86.

Conversely, the speed setting can be decreased by moving"=the '-'lever 96*clo'ckwise about pivot '98, *asviewed in-Fig. 1.- Moreover, it is to-"be"u'nder stood that when the-valve plunger 48- oscillates "or'reciprocates, it moves equal distances on'opposite sides of a neutral or closed port-position. When the ports 32 or-34 are open; they are wide "open. In other wordsythe entire-portarea is either connected to drain orto the pressure port and this is true regardless of Whetherthe pulse ratio be 50:50 or 1399. In this-manner with a constant areaopening, the fluid pressure pulses are determined by the length oftime the solenoids 50 or60 are energizecl'resulting in a plunger dwell at the port 'open positions-thereof.

It is to beunderstood of course-that the drum M is rotating at constant speed, as determined by the speedcontrol 92. If th'e'speed of "rotation of the drum T4 is at such a rate that contacts 82'] and 82 are successively engaged at every iortieth "-of a second, then the cycling of the pulses of "current-delivered to the solenoids '58 'and"6tl"wil1 "be in the order of C. P. S. each cycle including-one energization of each solenoid." Thatrate of rotation is mentioned as an example and it is obvious that the rate may be varied by in-- "creasing or decreasing as desired. Rotating under constant speed, the cycling of the solenoid energization is constant, and if the movable contact I04 engages the drum at the midpoint thereof the successive alternate pulses will be equal or in a 50:50 relation. Shift of the movable contact IMlengthwise of the drum will differentially alter the length of the pulses in either direction thru say 60:40, 70:30, and so on, until structure that there is a continual urge for movement in one direction, which necessitates a continued application of pressure in one expansion chamber or the other tomaintain the adjustable device in thestatus maintained condition.' Under such conditions a slight shift of the movable contact 04 is made'to ones'ide or the other of the medial-line'sfi in order tomaintain the apparatus in equilibrium position. That would incorporate a very slight difference" in pulse length as the drum contact-l4- is rotated,

which would be just enoughto allow sufficiently longer opening at one of the ports 32-, 3 3 to-provide the necessary sustaining pressure inone of the expansion chambers IE or I8 to hold the blade 22 in the status maintained'condition; I As a specific example, propellers manufactured: by one company, as a resultant of inherent factors, normally tend to shift toward a low pitchcondition while in operation, which gives rise tothe continued application of a relativelysmalrforce to the piston [4 which tends to move'the blade mechanism toward increase pitch. That force just balances the forces normally tending to shift the blade to low pitch. In" the illustrated embodiment this would mean application ofsufficient'counteracting pressure in the expansion chamber l8. *Therefore, the movable contact 10 3 in the equilibrium'position, that is; in the-status maintained position; a would 1 ride upon the drum 5 14 slightly to'-the=left -hand-side of=the=medial I r line 86' 'suchthat I there would ibel :delivered a pulse of' slightly greater length toa solerioid 58 'than' to solenoid 150. That would effect -:a:=;pulse ratio other than exactly-50:50 relation. (perhaps something like 51 to 49 as an assumed f'example) the longer duration of opening being for! theiport I 34.

One adaptation of the instant system oficon- *trol asapplied to rotating-structures of Lth'efpropeller type is schematically: illustratediin Fig. 2

-"where an engine. driven shaftZfi projects Ithru anti friction bearings l-supported by 'aniiaircraft structure I 32, and which'shaft' drivinglyiengages the hub 24 which is providedlwithrsockete I 3-4 for support ofthe variable pitch blade'si222.

In this instance the blades 22 are rotatable? in stack bearings-I thru the longitudinal move- '-ment of pistonl lrwithin the cylinder" Hand about a fixed spindle l 38 extending radiallyLfrom the hub 24. The-piston ishelically 'splined to the cylinder l2 at MEl and'tothe spindlefil38 at M2 such that pressure application inthelichamber IE or [8 effects radial movement of theipise ton M with respect to shaft 26, and effects ro- 'tary movement of'the'blade 22 thatdrives a bladegear I engaging a master or coordinating gear M5. Control passages 28 and '30-connect the expansion chambers lfiand l8 withthe ports32 and 34 of'th'e valve sleeve 36.

In this embodiment-the valve sleeve 36 with its plunger &8 and solenoids58,'60 are enclosed-withr'in a hydraulic regulator- I43 -made'up of an annular plate i550 and a cover assembly -l52*surrounding the shaft-'25 and rotatable-with the hub 24. Branches I54 and 156 from the passages 28 and 30 connect-with a decrease-pitch" port: and

' an increase pitch port respectively of a stand-by governor l 58, while a branch l joins the passage 40 extending from the supply port 38 of thelva'lve sleeve 35 to the pressure relief valve 4'! and the ""pump 42." In this'embodiment the'press'urerelief valve 41 has a feed back line I62 connecting with the increase pitch passageZS 0f alls'ervomotors such that the pressure potential'of the pressure source'line 40 is always capable of operating any device that is switched into'the circuit.

An extension- 164 from the pressure'source' line.

leads to a'feather control valvelfifi 'and controls the'charg-ing of an accumulator lfifl by means of a passage l in.

*Extending from the support 132' so as tobe' nonrotatable with respect to the'reg'ulator" I48 there tation of the propeller.

is an adapter assembly comprising a sleeve ll2 surrounding the" shaft 26 'and'providing a ring gear I14 inside of the regulator that meshes with 'a'pinion I10 for operating the pump 42"upon r0- J ournalled in the ring earlla are a" plurality of screw shafts' llsithat 'threadedly engage a'control ring that is -caiiable of movement longitudinally-of the shaft- 25.

Riding in a groove of the-control ringlflil there isa shoals? for control of the standby governor 158, as well as a shoe illl adapted to trip"the featheacontrol valve 1% when it is desired to either feather-orunfeather the propeller blades.

' The screw shafts' 'ilfi ar'e'drivenby an os'cillatable "ring-I86 thru its engagement witlrpi'nions lw "carried by the screw shafts, the ring-186 having an arm' lflfi linked by .192 to a manually operated controllevenw l. Oscillation of the lever lfi l about its pivot/"oscillates the ring F36"; and rotates the screw shafts l '18" which in turn moves the control ring' itil along the' shaft to' s'h'ift the position of the'shoes 182 and 184; The sh0 l82 moves a carriage !98--that niodifies in response with the standby governor I58, and if the carriage I98 is moved to the extreme left the standby governor is set for feathering shift so that tripping of the feather control valve I66 will allow a certain portion of the pressure from the accumula- J the blade in the feathered position. When the 'lever I94 is moved in the opposite direction such as to move the control ring I88 out of the extreme left hand position the feather control valve I66 will again be tripped and the standby governor I58 will be so modified as to allow a portion of the fluid pressure and the accumulator I63 to flow thru passages I76, I64 and 40, the branches I60 and I54 of the standby governor and control passage 28 and chamber It for moving the blades out of the feathered position.

i Movement of thecarriage I98 to the extreme right hand will so modify the action of the standby governor I58 that the fluid pressure from the pump will be applied to the decrease pitch side of the servomotor to effect negative pitch shift. An intermediate position of the control ring I80 so modified the action of the standby governor I58 that it will occupy the range of governed pitch relation, and may be used as a rough governor in such position if the solenoid actuating control valve is disconnected. There is such overlap of the valve lands and ports of the standby governor that there is a band of no response by the governor which is served by the solenoid valve.

Power is led to the solenoid actuated valve by means of brushes 200, 202 and29'1i supported by driven by a shaft H6 and pinion 2I8 engaging a gear 226 on the propeller shaft 26. The pilot's hand lever I94 has an arm 222 that is linked by 224' to the manual lever 95 for shifting the drum I4, whereby correlation is effected between the pulsing governor and the standby governor.

While the embodiment of the present invention as herein disclosed, constitutes a preferred form, it is to be understood that other forms might be adopted.

What is claimed is as follows:

1. In a fluid pressure system for varying the load of a prime mover for maintaining constant speed, the combination comprising, a servomotor reversibly operable by fluid pressure to vary said 7 load, a reciprocable valve having a pair of control ports connected to opposite sides of said servomotor for directing the flow of fluid pressure to opposite sides of said servomotor, means operable to continuously reciprocate said valve so that a succession of fluid pressure pulses are produced and delivered alternately to each side of the' servomotor, said control ports being fully opened during each reciprocable movement of said valve,

ration of the alternating pulses delivered to each side of the servomotor for maintaining the load against change, and means operable to differentially alter the duration of the alternating pulses delivered to each side of the servomotor for changing the load on the prime mover.

2. In a fluid pressure system for varying the l 8 7 load of a prime mover for maintaining constant speed, the combination comprising, a servomotor reversibly operable by fluid pressure to vary said load, a reciprocable valve having a pair of control ports connected to opposite sides of said servomotor for directing the flow of fluid pressure to opposite sides of said servomotor, means operable to continuously reciprocate said valve so that a succession of fluid pressure pulses are produced and delivered alternately to each side of the servomotor, said control ports being fully opened during each reciprocable movement of said valve, means operable to substantially means operable to substantially equalize the du- I equalize the duration of the alternatingrpulses delivered to each side of the servomotor for maintaining the load against change, means'operable to differentially alter the duration of the alternating pulses delivered to each side of the servomotor for changing the load on the prime mover and feed back mechanism operatively interconnecting the servomotor and said second and third recited means and operated in response to changing load for restoring the substantially equalized pulse duration when the changing load eiTects return to constant speed.

3. In a fluid pressure system, a fluid pressure operated motor, a control valve operatively connected with said motor for controlling the application of fluid pressure thereto, electromagnetic means operatively associated with said control valve for recurrently moving said control valve in opposite directions from a neutral closed posttion to alternately open said valve to apply fluid pressure impulses successively to one side of said motor and then to the other side, said electromagnetic means fully opening said valve during each recurrent movement thereof, and means operatively connected with said electromagnetic means to control the ratio of pulse duration that is applied to either side of the motor.

4.. In a fluid pressure system, a servomotor, a reversing valve operatively connected to said motor for controlling the application of fluid pressure thereto, said valve having a pair of op- I posed armatures thereon and a pair of springs engaging said vaive to normally maintain the said valve in its neutral closed position, electromagnets operatively associated with said armatures for attracting the same to open said valve in opposite directions, and means including a brush and alternately engaged commutating contacts connected each with the electromagnets for delivering a pulsating current to both of said electromagnets, alternately and equally to one and then to the other thereby to maintain rapid reciprocation of the control valve for equal and opposed pulse application to said motor with substantially no resultant motor movement, and

hold them in their neutral closed position, means 7 operable to impart to one of the coacting valve members a series of rapidly repeated impulses assure in opposite directions. from said neutral position; tending alternately toopen ,.said, valve to one side of said reversible motor anclthen to open, said valve to the other. side. iof saidtreversible motor, said one of the coacting valve members fully opening said valve during each of said im; p. pulses from the neutral position, and means. .op v erable to vary the ratio of the alternating impulses from an equal and Op ositerelationat which there is no resultant vmovement, of the servomotor, .to a relation of no impulses-in one direction from the .neutralposition and, con-.. tinuous impulse in the-opposite direction from said neutral position at which thereis -a .re sultant maximum-movementof the servomotor.

6. In a fluid pressure-systern i'or.varying, the pitch of-a blade in anengineepropeller comb e: tion, the combination comprising, a revers ble servomotor operatively connected to said blade for adjusting therpitchrof the blade in either increase or decrease pitch relation, a reversible slide valve operable to eontrel-theoperation of the servomotor by applying fiuid pressure to i r sid e eon e roma neti ansionerable to move theslide valve equal distances pnf' eitherrside of a neutra clos d os i crt open positionsto directfluid pressure to either side of t e si e erV9m tor-,: C y c nta ea s e ab t e er ize... t ev t ma net eans m an for d ivin -.t1 a eans a e constant speed, means including said rotary contact means for delivering a pulsating current to the electromagnetic means to effect alternate energization of said electromagnetic means for intervals of equal time duration while the blades are maintained against pitch change, means operable to efiect alternate energization of said electromagnetic means for intervals of unequal time duration to efiect a change in blade pitch, and means including mechanism operatively connected to said engine propeller combination and responsive to speed of rotation thereof automatically operable upon speed change of operation of the engine propeller combination for restorin the equal alternate energization of said electromagnetic means for establishing a nonchanging operation of the engine-propeller combination.

'7. In a fluid pressure system for varying the pitch of a blade in an engine-propeller combination, the combination comprising, a reversible servomotor operatively connected to said blade for adjusting the pitch of the blade in either increase or decrease pitch relation, a reversible slide valve operable to control the operation of the servomotor by applying fluid pressure to either side thereof, electromagnetic means operable to move the slide valve equal distances on either side of a neutral closed position to port open positions to direct fluid pressure to either m side of the reversible servomotor, rotary contact means operatively connected to said electromagnetic means and driven at constant speed for delivering a pulsating current to the electromagnetic means to alternately energize the electro- 5 magnetic means to effect fluid pressure pulse applications alternately to the opposite sides of the servomotor, and means including a manually operable member and a mechanism operatively connected with said engine propeller combination for shifting the rotary contact means from a position efiecting equal alternate pulse duration to a position for effecting unequal alternate pulse duration.

8. In a fluid pressure system for varying the pitchof, a propeller, blade to eifect substantially a constant, speed operation of an engine pro ll combination, and in which a double acting s h 74 motor responding topressure differenti- ,the blade in pitch angle, the combination icoinf' prising, a solenoid actuated reversely operable valve connected in the fluid pressure syster nffor ting fluid pressure to either" side Lof g 'j-i servomotor, means supporting the solenoid actu with said enginepropeller combination and rej:

sponding to propeller rotation for reversely pulsing the solenoid actuated valve ,to rapidlyl' and alternately deliver fluid pressure pulses to opposite sides of the servomotor piston, said N commutating means including a member piper-l: able thereon for rendering the alternatingpuls equal during on-speed operation of propellercombination, and means including said member for rendering the alternating puls e t a s. long a d s rt r e edtihe e r-T7 rective control.

9. In a fluid pressure system for varying the pitch of a propeller bladeto eifect substantliall constant speed operation of anengine propell; combinationand in which a double acting servo l" t rr eq li est pres ured f er nt a the blade in pitch angle; 'the combination com prising, a hydraulic regulator rotatable with the propeller and supplying a pressure source for the fluid pressure system, a solenoid actuated reversely operable control valve connected in the system for directing source pressure to each side of the servomotor, means operable to reversely energize the solenoid actuated valve to apply pulses of fluid pressure alternately to each side of the servomotor, means including a brush and a commutating drum operatively connected to said actuating means for equalizing the duration of solenoid energization and hence the alternately delivered pulses so that the servomotor is held against resultant movement, and means operatively connected with the engine propeller combination and responding to ofi-speed of the engine propeller combination for shifting the brush relative to the commutating drum to vary the energization of said solenoid actuated valve so that differential pulses are alternately applied to the opposite sides of the servomotor whereby the resultant movement of the servomotor overcomes the error in off-speed of the engine propeller combination.

10. In a fluid pressure system for varying the pitch of a propeller blade to effect substantially constant speed operation of an engine propeller combination, and in which a double acting servomotor responding to pressure difierentials shifts the blade in pitch angle, the combination comprising, a hydraulic regulator rotatable with the propeller enclosing the fluid pressure system and providing a source of fluid under pressure in said system, a reciprocating valve operatively connected to the source to the servomotor for applying pulses of fluid pressure alternately to one side and then the other side of the servomotor while the engine propeller combination is in operation, a pair of solenoids each operable to shift said reciprocating valve in an opposite direction, means operatively connected with said solenoids for alternately energizing the solenoids from a point outside of the regulator, means a 11 operatively connected with the, engine propeller combination and responsive to the speed of engine-propeller operation for proportioning the energization of said solenoids, whereby said solenoids receive substantially equal alternating pulses while the engine propeller combination is operating at on-speed and receive diflerentially unequal alternating pulses while the engine propeller combination is operating at an ofi-speed, and manual means operatively associated with said last recited means for selecting a speed setting for said engine propeller combination;

11. In a fiuid pressure system for varying the pitch of a blade in an engine-propeller combination having a fluid pressure source and a reversible servomotor operatively connected to the blade for adjusting the pitch position thereof, the combination of control means for exactly positioning said blade including an oscillatable valve open to said source and having a pair of control ports connected each with one side of said servomotor, electromagnetic means continuously oscillating the valve equal distances to either side of a neutral closed position for alternately opening each port to source pressure during operation of the engine propeller combination, a reference speeddevice, contact means operatively connected with the reference speed device and said engine propeller combination and operable thereby, including a brush and a commutating drum operatively connected to said electromagnetic means for energizing said elec-*' tromagnetic means to effect equal alternate fluid pressure pulse application to opposite sides of said servomotor by said valve for maintaining a selected blade pitch position, means operatively connected with said contact means for shifting the commutating drum relative to the brush to differentially alter the length of fluid pressure pulses applied by the valve to effect a change in the pitch position of said blade, and means operatively connected with said engine propeller combination and the brush and responsive to a speed change in the operation thereof for shifting the brush relative to the commutating drum I to restore the equal and alternate fluid pressure pulse application to opposite sides of said servomotor for maintaining the new pitch position of the blade.

References Cited in the file of this patent UNITED STATES PATENTS 7 Date 

